The inability of the present therapies to mitigate the devastating effects of sepsis and multiple organ failure in the critically ill patient indicates that more knowledge of the pathophysiology of sepsis is needed if we are to develop better, more effective interventions. In this respect, we and others have found that lymphoid apoptosis (Ao) is increased in a variety of tissue sites. Further, those lymphocytes that do not appear overtly apoptotic are nonetheless more susceptible to Ao upon subsequent activation. A similar condition seems to be evident in various septic mouse tissue macrophages. While we have found that divergent mediators such as steroids and TNF can contribute to these apoptotic changes, the inhibition of FasL-Fas signaling by either FasL gene deficiency or delayed (12 h post-sepsis) treatment with FasL binding protein (FasFP), not only protected mucosal immune cell populations from Ao but reduced the morbidity and mortality seen in sepsis. This was not seen with TNF inhibition. Further, studies indicate that FasL may induce Ao via either a caspase-8 mediated caspase cascade and/or the Bid induced mitochondrial pathway of Ao. These observations are in keeping with the findings of others that pan-specific caspase inhibition or the over-expression of Bcl-2 can also serve to protect these animals from the sequelae of sepsis. However, much remains to be understood, about FasL-Fas mediated Ao in the septic state and the consequences of its activation. With this information in mind, we put forward the following hypothesis: that the induction of FasL induced Ao is reflective of an aberrant response to septic stimuli that contributes to both immune as well as possible non-immune cell dysfunction and eventual mortality associated with septic insult. To examine this we have designed the following aims: 1) We will determine the nature of the regulatory mechanisms that contribute to increased FasL-Fas induced lymphocyte/macrophage Ao initially by comparing the capacity of anti-FasL/-Fas/-FADD/-Caspase-8/-Bid, etc., siRNA to inhibit the onset of Ao and death associated with sepsis. We will also determine if FasL-Fas mediated Ao proceeds through either the caspase induced Type I vs. Bid induced-Type II death receptor activation pathways. We will assess the roles of IL-10, IL-16, IL-18 and NO (from iNOS) in potentiating FasL-Fas mediated Ao following sepsis. 2) We will assess the contribution of endoplasmic reticular /caspase-12 mediated apoptotic process to the onset of FasL-Fas dependent and/or independent sepsis induced Ao. 3) We will test the hypothesis that sepsis induced tissue/organ injury may be an indirect result of dysfunctional apoptotic cell clearance. Such data will provide not only new insight into the pathobiology of sepsis, but also better therapeutic targets for the management of this devastating condition.